Air quality in parts of Sydney have reached "hazardous" levels as smoke and fog blankets the city, again prompting health warnings. The combination of fog and residual smoke from hazard reduction burns in the Blue Mountains earlier this week caused visibility issues on Friday morning, particularly in Sydney's west. "It's due to a lack of wind being able to move some of that air out and the wind has been trapped in the basin," Bureau of Meteorology forecaster Chris Ryan told AAP. Conditions are expected to slowly change over the coming days as a tropical front moves over the southern parts of NSW, Mr Ryan said. Some parts of Sydney's east and northwest recorded "hazardous" air quality on Friday morning, including Richmond, Macquarie Park and Lindfield. Authorities are urging older people, children, and anyone with heart or lung disease to avoid exercising outdoors and stay inside as much as possible. Everyone else is advised to reduce prolonged or heavy exertion outdoors. The NSW Rural Fire Service said no additional fires have been lit in the past 48 hours and no further burns are scheduled in Sydney on Friday. The heavy fog at Windsor affected some train services on the western and north shore lines. Australian Associated Press